1. Field of the Invention
The present disclosure provides optical sensors operating with resonant leaky modes in periodic structures where angular, spectral, modal, and polarization diversity is advantageously applied for high-precision sensing in compact systems formats. Cross-referenced data sets thus obtained, fitted to numerical models, provide added degrees of precision and accuracy to enhance the quality of the sensing operation in a broad variety of applications.
2. Description of the Related Art
Numerous optical sensors for bio- and chemical detection have been developed commercially and in the research literature. Example devices include the surface plasmon resonance sensor, MEMS based cantilever sensors, resonant mirror, Bragg grating sensors, waveguide sensors, waveguide interferometric sensors, ellipsometry and grating coupled sensors. Of these, although dramatically different in concept, function, and capability, the surface plasmon resonance (SPR) sensor comes closest to the guided-mode resonance (GMR) sensor that is the subject of this disclosure. Both GMR and SPR sensors provide tag-free biochemical detection capability.
The term surface plasmon (SP) refers to an electromagnetic field induced charge-density oscillation that can occur at the interface between a conductor and a dielectric (for example, gold/glass interface). An SP mode can be resonantly excited by parallel-polarized TM polarized light (TM polarization refers to light with the electric field vector in the plane of incidence) but not with TE polarized light (the TE polarization refers to light where the TE vector is normal to the plane of incidence). Phase matching occurs by employing a metallized diffraction grating, or by using total internal reflection from a high-index material, such as in prism coupling, or an evanescent field from a guided wave. When an SPR surface wave is excited, an absorption minimum occurs in a specific wavelength band. While angular and spectral sensitivity is very high for these sensors, the resolution is limited by a broad resonant linewidth (˜50 nm) and signal to noise ratio of the sensor response. Furthermore, as the operational dynamic range of the sensor is increased, the sensor sensitivity typically decreases. Since only a single polarization (TM) can physically be used for detection, change in refractive index and thickness cannot simultaneously be resolved in one measurement. This is particularly important in chemical sensor applications where binding kinetics include thickness changes at the sensor surface, while background refractive index can vary depending on analyte concentration. The disclosure provided herein can remedy some of the limitations of the present art.
Magnusson et al. discovered guided-mode resonance filters that were tunable on variation in resonance structure parameters. Thus, spectral or angular variations induced via layer thickness change or on change in refractive index in surrounding media or in device layers can be used to sense these changes. Wawro et al. discovered new GMR sensor embodiments as well as new possibilities of applications of these when integrated with optical fibers. There are also additional aspects of GMR sensors in various application scenarios.